This invention relates to methods and apparatus for producing extruded membranes, and in particular relates to methods and apparatus to produce extruded polymeric membranes, and inparting a curvature during the extrusion process. Moreover this invention relates to extrusion dies having a curved die slot for imparting a curvature to the extruded membrane. Finally this invention relates to articles or products produced by the apparatus and methods, such products incorporating seamless curvate structure or structures.
There are various methods and apparatus for producing extruded polymeric membranes. Such membranes are generally thin and have negligible bending stiffness but have substantial longitudinal, traverse and shear modulus. The negligible stiffness allows for relatively easy and repeatable isometric mappings.
Such prior art devices generally illustrate the production of articles having a planar structure. It is desirable, however, to produce articles having curvate structure. Moreover it is desirable to extrude polymetric membrane so as to impart complex curvature during the extrusion process where the complex curvature can be imparted in three dimensions such as along X, Y, Z axis.
Generally speaking the X-axis or dimension is the xe2x80x9cMachine Directionxe2x80x9d which is the direction of undisturbed material flow away from the die exit of an extrusion die. Furthermore the Y-axis generally relates to the xe2x80x9cTransverse Directionxe2x80x9d which is the local reference direction orientated normal to X and along the width of the slot die. The Z -axis or direction is known as the xe2x80x9cNormal Directionxe2x80x9d and is the local reference direction normal or at 90xc2x0 to both X and Y orientated across the die gap.
The X, Y, Z axis or directions comprise a local or orthonormal coordinate frame. The frame is orientated locally with the extrusion die slot and typical dimensions by way of example are in the vicinity of:
X=infinite
Y=up to 25 feet or more
Z=0.001 to 0.050 inches
such dimensions are given by way of example only and the invention described herein is not limited thereto.
The following terms have the following definitions herein:
Curvature. A quantitative measure along a space curve of the change in direction of the tangent vector differentiated along the curve length.
Plane curve. A curve that can be described in 2 dimensions, the vector product of any 2 distinct curvature vectors on the curve would always be normal to the plane of the curve.
Normal Curvature. A measure of curvature on a surface of a space curve measured normal to the surface and along the spacer curve at any point where the curve and the surface are tangent.
Gaussian Curvature. A precise quantitative measure of the intrinsic curvature at a point on a surface. It is the product of the minimum and maximum normal curvatures at any point on a surface.
A flat piece of paper has a Gaussian curvature of 0. A sphere has a Gaussian curvature of the inverse of the radius squared.
Complex Curvature. A qualitative definition of a surface where at least some portion of its area cannot be made flat.
Isometric Mapping A bending operation performed on a flexible surface such that the length along any trajectory across the surface remains unchanged before and after the bending operation. As a corollary the surface does not experience any shear strain. As another corollary Gaussian curvature is preserved at all point on the surface.
Membrane. A thin flexible surface that has negligible bending resistance, but substantial longitudinal, transverse and shear moduli.
Isotropic elasticity. An elastic material where the modulus of elasticity is constant and uniform in all directions.
Anisotropic elasticity. An elastic material where the modulus of elasticity varies uniformly as the direction of analysis is changed.
Moreover membranes can generally be described as thin flexible surfaces that have negligible bending resistance but substantial longitudinal, transverse shear moduli.
Current methods for producing polymeric membranes having a curvature include first extruding, casting, calendaring, or laminating a flat membrane and then imparting shape by thermo forming continuous membranes or by xe2x80x9cpaneling and joiningxe2x80x9d a plurality of flat membrane pieces to produce a finished membrane with complex curvature.
The paneled method includes flat membrane pieces that are reinforced while the thermo forming method also allows for reinforcing the finish membrane after thermo forming.
Other methods and devices are shown in U.S. Pat. No. 4,906,171, which teaches a die head that can pivot with respect to the internal source of extrudate material. Such pivoting allows for the adjustment of the distance, and hence the resistance to flow, from the internal source to the different locations across the die exit profile. This causes the extrudate to have different velocities at different locations across the die exit and as a result causes the extrudate to curve. This method will allow the extrusion of a section with curvature that varies along its length. In the case of a thin film, this method could produce an extrudate with curvature about only the normal direction Z. However this method would not permit one to vary the shape of the film across the width of the film (Y), such as to make films with programmed complex curvature.
Moreover U.S. Pat. No. 4,237,082 teaches the control of an extrusion die exit rate, where the rate of extrusion of an elongated article from a dynamic extruder is measured, and a measurement signal representative thereof is established. The measurement signal is then compared with a set point signal representing the desired extrusion rate to produce a control signal which is responsive to the difference between the measurement signal and the set point signal, to control the rate and pressure of extrudate to the manifold of the die.
Moreover U.S. Pat. No. 3,920,365 illustrates apparatus for making polymeric films whereby flowable polymeric material is extruded in molten form through an extrusion die orifice in the form of a web of film and which apparatus includes means for selectably controlling either by heating or cooling temperature of the isolated parts of the die lips defining the die orifice to improve thickness uniformity of the film.
U.S. Pat. No. 4,851,173 relates to a process for the continuous preparation of an oriented film of a polymer material, to produce highly oriented films from ultra high molecular weight (UHMW) polyethylene, polyester or the like.
Moreover U.S. Pat. No. 4,966,807 describes a method of extruding films through a counter-rotating annular die, where the extrudate is a thermotropic liquid crystal polymer that is given biaxial liquid orientation by the shear included between the walls of the counter-rotating dies.
Furthermore U.S. Pat. No. 5,333,568 teaches material for the fabrication of sails, where fine filaments of various high performance fibers are pull-truded and laminated, between films to produce a thin lightweight film.
Also some polymer, notably polyethylene are cross linked when exposed to ionizing radiation as taught by the article xe2x80x9cPolymer The Origins and Growth of a Sciencexe2x80x9d, by Herbert Morawetz, published by John Wiley and Sons 1985, pages 180 and 181 Dover (1995) edition.
Furthermore U.S. Pat. No. 4,221,753 teaches an extrusion process wherein coolant under pressure is directed under the outer surface of a thermo plastic material being forced through an extrusion orifice such that at least a portion of the coolant immediately vaporizes and the resultant heat evaporation required for such action is taken from the material immediately contacting same. The coolant is introduced into a intermediate zone down stream from an initial extrusion zone and the interface between such zones is insulated so as to restrict conductive heat transfer between such zones immediately adjacent thereto so as to prevent possible freeze ups of the material in the initial extrusion zone.
U.S. Pat. No. 5,770,129 illustrates apparatus and method for controlling mass flow in an extrusion die that provides cross-directional measurements of at least one property of an extrusion produced by the extrusion die, such measurements being made at a plurality of cross-directional locations that successively span the extrusion. The measurements are assembled to provide cross-directional profiles of the extrusion.
Moreover U.S. Pat. No. 4,423,191 teaches a method for curing a resinous composition comprising an admixture of thermo setting resin and dialectically loss particles. Furthermore shaped articles of manufacture made by curing resinous compositions are disclosed by the method of U.S. Pat. No. 4,423,191.
Furthermore EP 0056657 illustrates a sail for sailing craft and method of making the sail.
Moreover U.S. Pat. No. 5,061,553 describes an article of manufacture namely a sail comprised of a plurality of flat co-extruded polymeric sheets.
Furthermore U.S. Pat. No. 5,259,747 relates to apparatus for providing uniformity of web thickness by providing a plurality of gap-controlling thermo energy sources, and heat exchange communication with the thermal energy sources, and assembly for compensating for temperature variation proximate to the thermal energy source and across the apparatus width.
Moreover U.S. Pat. No. 4,744,745 relates to a variable contour rubber extruder for continuously extruding rubber having compositions for various uses through a mouthpiece with belts and more particularly to a rubber extruder capable of varying cross-section of a mouthpiece to obtain particular contours of extruded rubber.
Moreover U.S. Pat. No. 3,940,221 relates to a die having controllable lip openings, through which flowable material is forced to form a sheet, film or tube. Lip opening control means include an elongated member which expands or contracts thermally and adjusts the lip opening to control the thickness and heating means is provided for controlling this expansion and contraction.
It is an object of this invention to provide an improved method and apparatus for producing extruded membranes having a curve imparted in a transverse direction relative to the flow of extruded material.
It is a further object of this invention to provide an improved method and apparatus for producing films having complex curvatures.
Furthermore it is an object of this invention to produce continuous seamless extruded articles having complex curvatures.
It is an aspect of this invention to provide a method of producing an extruded membrane of solidified flowable material having a desired curvature comprising: extruding flowable material through a die slot having a curved section; measuring a characteristic of said extruded material at a plurality of positions along said die slot; comparing said plurality of measurements against a desired plurality of measurements respectively at each said position along said die slot and generating a plurality of signals respectively in response thereto; effecting a change in said measured characteristic of said extruded material at said positions along said die slot in response to said signals such to produce said extruded material having a desired curvature.
It is another aspect of this invention to provide a method of producing an extruded membrane of solidified flowable material having a desired curvature comprising: extruding flowable material through a die slot having a curved section; measuring the velocity of said extruded material at a plurality of positions along said die slots; comparing said plurality of velocity measurements against a desired plurality of target velocity measurements to produce an error signal; changing the velocity of the flowable material at each said plurality of positions in response to said error signal; such to produce an extruded membrane having a desired curvature.
It is yet another aspect of this invention to provide an extrusion die for extruding a flowable material to produce a membrane having a desired curvature comprising die slot means having a curved section; a plurality of measuring means disposed along said die slot means for measuring a selected characteristic of said extruded material; means for varying the characteristic of said extruded material in response to said plurality of measurements so as to produce said extruded membrane having a desired curvature.
Furthermore it is an aspect of this invention to provide a sail comprising a seamless extruded material having a desired three-dimensional curvature.
It is also an aspect of this invention to provide a pool liner comprising a seamless extruded material having a desired three-dimensional curvature.